Infrared radiator that is designed as surface radiator

ABSTRACT

An infrared irradiating heater having a radiating body with a housing comprised of a ceramic and having a planar radiating surface, a multiplicity of substantially flame-free passages extending perpendicular to the surface and opening at the surface, and a rear surface, the passages extending to the rear surface, the passages having lengths less than 300 mm, the total cross sectional area of the passages at the planar radiating surface being in a ratio to the area thereof in excess of 50%, and the passages having length to maximum diameter ratios of at least 5. A burner plate spaced from the rear surface defines a combustion chamber with it so that the combustion is effected substantially only in this combustion chamber and the passages are free from flame and serve as radiator surfaces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/EP99/10034 filed Dec. 17,1999 and based upon German application 199 01 145.1 filed Jan. 14, 1999under the International Convention.

FIELD OF THE INVENTION

The invention relates to an infrared radiator configured as a surfaceradiator with a radiating body which, at its rear side, is heated by aburning fluid-air mixture and whose front surface emits the infraredradiation.

STATE OF THE ART

Infrared radiators configured as surface radiators are used in knownmanner in dryer systems for the drying of web shaped materials, forexample, paper webs or cardboard webs. Depending upon the width of theweb to be dried and the desired heating power, the requisite number ofradiators with flush emitting surfaces are assembled into a drying unit.

In the publication “Radiant efficiency and performance considerations ofcommercially manufactured gas radiant burners (Speyer et al., Exp. HeatTrans, 9, 213-245, 1996), various types of gas heated infrared radiatorsare compared with one another. A radiator is proposed which, amongothers, has a ceramic plate provided with holes through which a gas/airmixture flows and which burns on its surface. To avoid a migration ofthe flame and to increase the radiation efficiency, a metal grid isarranged ahead of the ceramic plate.

This known principle, which is used by many manufacturers, has thedrawback that the radiation efficiency is comparatively small because ofthe low emission coefficient of the ceramic plate at high temperatures.In addition, the metal grid has only a limited life when the radiator isoperated at high powers.

OBJECT OF THE INVENTION

The object of the invention is to provide an infrared radiatorconfigured as a surface radiator which has a high efficiency attemperatures above 1100° C. and a long operating life.

SUMMARY OF THE INVENTION

This object is achieved with an infrared radiator configured as asurface radiator with a radiating body (15) which is heated at its rearside by a burning liquid/air mixture and from its front surface emitsthe infrared radiation. According to the invention the radiating bodyincludes a multiplicity of throughgoing passages functioning as hollowspace irradiators, in which the wall area/cross sectional area ratio inthe flame-free region is greater than 10, preferably greater than orequal to 20.

Advantageously the passages are of circular cross section or areconfigured in the form of regular polygons whereby the length/maximumdiameter ratio in the flame-free region is greater than 3, preferablygreater than or equal to 5.

The radiating body can be constructed from a row of plates arranged in aspaced relationship to one another, whose intervening spaces form thepassages, whereby the height of the plate/spacing between neighboringplates form a ratio in the flame-free region which is greater than 3,preferably greater than or equal to 5.

The proportion of the opening area of the passages to the total area ofthe front side of the radiating body amounts to at least 30%, preferablymore than 50%.

The radiating body is preferably fabricated from ceramic.

The passages can have a depth less than 300 mm, preferably between 10 mmand 100 mm.

Advantageously the passages have a cross section widening toward thefront side.

A burner plate can be spaced from the radiating body to form acombustion chamber therewith.

The radiating body can be made from a silicon carbide reinforced withcarbon fibers.

The infrared body is preferably used for drying of web-shaped materials,especially paper webs or cardboard webs.

The invention makes use of the physical effect that a channel forminghollow radiator has at its opening an emission factor which increaseswith its ratio of wall area/cross sectional area. With a wall area/crosssectional area ratio greater than or equal to 20, a channel shapedhollow chamber radiator can have an emission factor of approximately 1when it is fabricated from a ceramic with an emission factor of about0.5.

BRIEF DESCRIPTION OF THE DRAWING

The drawing serves to elucidate the invention based upon embodimentsshown in a simplified manner. In the drawing:

FIG. 1 is a cross section of the basic construction of an infraredradiator;

FIG. 2 is a plan view of the radiating front side of a radiation body;

FIG. 3 a section through the radiating body of FIG. 2;

FIGS. 4 to 7 are respective plan views of the radiating front side ofdifferent embodiments of a radiating body with tubular channels; and

FIGS. 8 and 9 are diagrams of in infrared radiator with slip shapedchannels in the radiating body.

MANNER OF CARRYING OUT THE INVENTION

The infrared radiator according to the invention is preferably heatedwith gas. Alternatively heating with a liquid fuel as heating fluid ispossible.

As shown in FIG. 1, each radiator includes a mixing pipe 1 into which amixing nozzle 2 is screwed at one end. A gas feed line 3 is connected tothe mixing nozzle 2 and is connected with a manifold 4 from which aplurality of mutually adjacent radiators are supplied with gas 5.

The supply of air is effected via a hollow traverse 7 on which themixing pipe 1 is fastened. The connecting duct 8 for the air feed opensin the upper part of the mixing pipe 1 into a downwardly open airchamber 9 which surrounds the outlet ends of the mixing nozzles 2 sothat in the mixing chamber 10 of the mixing pipe 1 a gas/air mixture isintroduced from above.

At the lower open end of the mixing pipe 1, a housing 11 is fastened inwhich a burner plate is arranged. The burner plate 12 has a row ofthroughgoing bores 13 which open into a burner chamber 14 which isformed between the burner plate 12 and a radiating body arrangedsubstantially parallel to the burner plate 12 but spaced therefrom. Themixing pipe 1 opens into a chamber sealed off by a hood 16 which isclosed at its other end by the burner plate 12. To distribute thegas/air mixture uniformly on the backside of the burner plate 12, abaffle plate 18 is arranged in the mixture distribution chamber 17 andthe supplied mixture flows against it. The burner plate 12 and theradiating body 15 are fitted into the housing in a peripherallycontinuous refractory seal 19 which laterally closes the combustionchamber 14.

The radiating body 15 is preferably fabricated from ceramic, especiallyaluminum oxide or zirconium oxide, aluminum titanate, corundum ormullite. Silicon carbide has been found to be especially suitable,particularly when it is reinforced with carbon fibers.

Alternatively, the radiating body 15 can also be fabricated from aheat-resistant metal.

It is important for the invention that the radiating body 15 contain amultiplicity of throughgoing passages 20 which are effective as hollowspace radiators. The passages 20 are heated at the rear side of theradiating body 15 which bounds the combustion chamber 14 and aresubstantially flame-free; the gas-air mixture burns essentially only inthe combustion chamber 14. So that the passages 20 as hollow spaceradiators will have a high emission factor, the ratio of their areas totheir cross sectional areas is, in their flame-free regions, greaterthan 10 and preferably ≧20.

The passages 20 are either tubular (FIGS. 2 to 7) or slit shape (FIG.8). The cross section of the tubularly-shaped passages is preferablyeither circular or in the form of a regular polygon. Withtubularly-shaped passages 20, the length/maximum diameter ratio in theflame-free region is greater than 3 and preferably is greater than/equalto 5. Alternatively, the passages 20 can also be configured asslit-shaped as shown in FIG. 8. Preferably with this embodiment of theradiation body, the radiation body 15 is constructed from a row ofspaced-apart plates 21 whose intervening spaces form the slit-likepassages 20. The spacing of two neighboring plates 21 is in a ratio tothe lengths of the plates 21 in the flame-free region which amounts, inthis embodiment, to greater than 3, preferably greater than/equal to 5.The lengths of the passages 20 are, in all embodiments, measured fromthe heated rear side of the radiation body 15 in the direction towardthe radiating front surface; in FIG. 1 it is measured from abovedownwardly. The lengths of the passages 20 amounts to less than 300 mm,preferably toward 10 mm to 100 mm. In the exemplary embodiment thelength amounts to about 40 mm.

So that higher efficiency can be achieved, at the front side of theradiation body 15 shown in the lower part of FIG. 1, the proportion ofthe opening area of the passages 20 serving as radiation surfaces of theentire area of the front side is at least 30%; preferably the proportionof the opening area amounts to more than 50% of the total area of thefront side.

Preferably the passages widen toward the rotating front side as is shownin FIG. 3. A diffuser-like widening of the passage 20 effects a moreuniform heat distribution and reduces thereby stresses in the radiatingbody 15.

The combustion chamber 14 ensures that the combustion will occur overthe entire rear side area of the radiating body 15. The flame canpropagate laterally. In an alternative embodiment without a separatecombustion chamber, the passages 20 are connected together at the rearside of the radiating body 15 by transversely running passages. Theflames burn, in this embodiment, at the inlet portion of the passages 20at the rear sides of the radiating body 15 whereby transverse passagesensure uniform distribution of the flames over the entire back side ofthe radiating body 15. In this embodiment the values of the areaproportions or length proportions of the passages pertain to theflame-free portions.

With all of the radiating bodies 15 shown in the Figures, the radiatingfront side is about 200 mm in width and about 150 mm in height.

In FIGS. 2-7 various embodiments have been shown of radiating bodies 15with throughgoing passages 20. The cross section of the passages 20 iseither circular in the form of a regular polygon. The ratio of thelength to the maximum diameter of the passages in the flame-free regionamounts to more than 3 and preferably is greater than or equal to 5.

In the embodiment according to FIGS. 2 and 3, the passages are soconfigured that they widen from a circular cross section to about 4 mmin diameter to a square opening area with a side length of about 8 mm.The passages 20 are so arranged in a uniform pattern over one anotherand adjacent one another that on the front side webs of about 2 mm inthickness remain.

In the embodiment of FIG. 4, the mouth openings of the passages 20 arecircular with a diameter of about 5 mm. The walls around the mouthopenings of the passages 20 are circular. In order to have the passages20 as densely packed as possible, they are arranged in a face-centeredpattern. In the embodiment of FIG. 5, they widen over their entirelengths in circular cross section passages with a diameter to about 4 mmto a mouth diameter of about 15 mm. The result is fewer passages 20 witha larger mouth diameter than with the embodiment according to FIG. 4.

FIGS. 6 and 7 show radiating bodies in which the passages are of squarecross section (FIG. 6) or hexagonal cross section. The overall radiatingbody 15 is honeycomb-shaped with throughgoing passages 20.

FIGS. 8 and 9 show a radiating body which has a row of slit-likepassages 20. The slit-shaped passages 20 extend preferably over theentire width of the radiating body 15. They are preferably so producedby arranging a row of plates 21 of ceramic with spacings from oneanother. The intervening spaces between the plates 21 in thisembodiment, the plates 21 are so arranged that the ratio of the heightof the plate 21 to the distance between two neighboring plates 21 in theflame-free region is greater than 3 and is preferably greater than orequal to 5. The heights of the plates 21 are defined in the radiatingdirection and thus in FIG. 1 run from top to bottom.

The construction of an infrared radiator with such a radiating body 15has been illustrated in a partial view in FIG. 9.

The housing 11 is comprised of a metal holder frame which, on eachlongitudinal side, holds a respective ceramic bar 22. Each of theceramic bars is formed on the respective inner side with slit-shapedopenings in each of which a ceramic plate 21 is inserted with itslateral end and is thus held. In the view of FIG. 9, the plates 21forming the radiating body are arranged above one another and below oneanother. The radiating body 15 emits the infrared radiation downwardly.A second metallic holding frame 23 holds the burner plate 12 which hasonly been indicated diagrammatically in FIG. 9. The burner plate 12contains a row of bars 13 which open into a combustion chamber 14 as hasalready been described in elucidation of FIG. 1.

The embodiment according to FIGS. 8 and 9 has an advantage that thepassages are formed from simply shaped plates 21. They can thus befabricated from a temperature-resistant and stable material even whenthe same may be difficult to shape and/or to machine. An especiallysuitable material for the plates 21 has been found to be silicon carbidewhich has been reinforced by carbon fibers.

Based upon the possibility of using it at temperatures above 1100° C.,its high specific power density and its long life, the infrared radiatorof the invention is especially suitable for the drying of web-shapedmaterials at high speed. A preferred field of use is in the drying oftravelling paper webs or cardboard webs in paper-making factories,especially downstream of coating units.

What is claimed is:
 1. An infrared irradiating heater for drying paperand cardboard webs, said heater comprising: a housing; a radiating bodyin said housing comprised of a ceramic and having a planar radiatingsurface, a multiplicity of substantially flame-free passages extendingperpendicular to said surface and opening at said surface, and a rearsurface, said passages extending to said rear surface, said passageshaving lengths less than 300 mm, the total cross sectional area of saidpassages at said planar radiating surface being in a ratio to the areathereof in excess of 50%, and said passages having length to maximumdiameter ratios of at least 5; a burner plate in said housing spacedfrom said rear surface and defining a combustion chamber therewith, saidburner plate being provided with throughgoing bores opening into saidcombustion chamber; a peripherally continuous seal extending aroundperimeters of said burner plate and said radiating body and sealing saidcombustion chamber so that combustion in said heater is substantiallyconfined to said combustion chamber; a distribution chamber formed insaid housing along a side of said burner plate opposite said combustionchamber for distributing a fuel/air mixture to said bores; and a mixingpipe supplied with fuel and air opening into said distribution chamber.2. The infrared irradiating heater defined in claim 1 wherein saidradiating body is composed of a ceramic selected from the group whichconsists of aluminum oxide, zirconium oxide, aluminum titanate,corundum, mullite and graphite-reinforced silicon carbide.
 3. Theinfrared irradiating heater defined in claim 2, further comprising abaffle in said distribution chamber ahead of an outlet for said pipe todistribute said mixture in said distribution chamber.
 4. The infraredirradiating heater defined in claim 3 wherein said passages are ofcircular cross section or of regular polygonal cross section.
 5. Theinfrared irradiating heater defined in claim 3 wherein said passages aredefined between a plurality of plates.
 6. The infrared irradiatingheater defined in claim 3 wherein said passages have lengths of 10 mm to100 mm.
 7. The infrared irradiating heater defined in claim 6 whereinsaid passages have lengths of about 40 mm.
 8. The infrared irradiatingheater defined in claim 7 wherein said passages have cross sectionswidening toward said planar radiating surface.